German Published Patent Document No. 30 24 209 and German Published Patent Document No. 41 09 498 discuss a method for the liquid-cooling of internal combustion engines in which the setpoint value of the coolant temperature is varied as a function of different parameters such as outside temperature, operating state of the engine, etc. This makes it possible to quickly attain the operating temperature after startup of the engine, while preventing the engine from overheating in all operating states. However, changing the setpoint value of the engine temperature also affects the operating performance of the engine, making it necessary to perform additional optimization.